Despite the introduction in recent years of several new diagnostic imaging modalities, traditional methods of producing diagnostic images, such as transmission radiography and fluoroscopy, remain popular, as they are cost-effective and diagnostically useful. In both radiography and fluoroscopy, a source of penetrating energy, such as X-rays, is directed to illuminate a volume of interest within the patient. An imaging receptor, such as photographic film or an image intensifier, is positioned opposite the source to receive the penetrating energy transmitted through the volume of interest. Differential attenuation by various elements of the volume of interest, due to variations in the length of the imaging energy path through the object, and in the density of the material along that path, results in corresponding amounts of imaging energy striking the various locations of the imaging receptor. By recording the amount of imaging energy striking such locations, an image corresponding to structures internal to the volume of interest may be produced.
When X-rays or other potentially dangerous sources of penetrating energy are used to produce an image, it is essential that the dose to the patient be minimized. At the same time, it is an objective to acquire an image of high diagnostic quality. Typical imaging receptors include photographic film, electronic image-intensifier/camera chains, and solid state imaging receptors. Although the characteristics of these receptors vary, all of them have limited dynamic range, and overexposure or underexposure may produce a poor quality or unusable image. Accordingly, in order to produce a high-quality diagnostic image, while minimizing the dose to the patient, proper selection of exposure parameters is essential. An improper exposure resulting in a diagnostically unusable image, is particularly disadvantageous, because repeating the examination necessarily involves additional exposure of the patient to X-rays or another energy source.
Historically, radiologists or skilled examination technicians have selected the exposure parameters based on observed or measured characteristics of the patient volume being imaged (e.g., the thickness of the portion of the patient to be imaged), with the help of published tables and empirically derived knowledge of how particular anatomical features should be exposed for best results. A number of disadvantages result from the selection of all exposure parameters solely by human operators, because the operators may make mistakes. For example, the operator may erroneously determine or observe a physical parameter of the patient or the imaging path, erroneously select a parameter from a publication or table, or make a calculation error.
A number of automatic exposure control (AEC) systems have been developed for use in improving exposure control in radiography applications. Conventional AEC systems allow the operator to specify a desired exposure (or dose) in terms of the dose rate integrated over time. A sensor is positioned near, and typically in front of, the image receptor to measure the imaging energy incident thereon throughout the examination. During the exposure, the AEC system determines the accumulated exposure amount (for example, by integrating the instantaneously-measured dose rate over time) and terminates the exposure when the accumulated value reaches that specified by the operator. Such systems are sometimes referred to as "phototimer detectors."
Similarly, automatic brightness systems (ABS) have been developed for use in improving exposure control in fluoroscopy applications. Conventional ABS systems attempt to control the instantaneous exposure rate to achieve a consistent, predetermined exposure level (or "brightness"), averaged across the image. If an image intensifier (or some other image receptor which produces a physically observable image) is used, a sensor may be provided to observe the image screen and directly measure its brightness. Alternatively, an output signal from the image receptor may be used to measure the image brightness. The ABS uses the image brightness measurement to control an exposure rate parameter of the imaging energy source as necessary to achieve the desired average brightness over the image. Conventional ABS systems used in conjunction with fluoroscopy systems, in which the source is an X-ray tube, typically control the X-ray tube high-voltage. However, other parameters may also be used to control the exposure rate, and still other parameters could be used to adjust the sensitivity or dynamic range of the imaging receptor.
Although both AEC and ABS systems have been advantageously applied to reduce patient exposure and improve diagnostic image quality, existing systems exhibit several deficiencies. In most imaging applications, the region of diagnostic interest to the physician does not fill the entire image field. It is often difficult in advance to precisely position the patient with respect to the imaging equipment such that an anatomical feature or region of interest lies entirely within the image field and substantially fills the field. Accordingly, the image field is typically selected to be somewhat larger than the feature of interest so that the entire feature will lie within the image even if initial misalignment or subsequent patient movement occur. Existing AEC systems in which an ion chamber or similar exposure sensor is used typically measure exposure rate or image brightness at one, two, or three predefined locations within the image field. Existing ABS and AEC systems in which an image intensifier is used as the imaging receptor typically measure exposure rate or image brightness over portion of the image field ranging from 30-60 percent of the area of the field. For example, phototimer detectors are frequently used individually, or in pairs or triplets, to sample several areas of the radiographic field. In systems designed for PA chest imaging (in which a front-to-back chest image is desired), paired detectors may be used.
An exposure measurement error will occur whenever the anatomical structure or region of interest does not completely cover the entire field of the exposure measuring sensor. The size and locations of the measuring fields in conventional AEC/ABS systems are fixed. It may be difficult or impossible prior to exposure to position the patient such that the anatomical structure or region of diagnostic interest accurately corresponds to the measurement location or locations used by the AEC/ABS systems. This is especially the case if the anatomical structure or region of interest is irregularly shaped or is small with respect to the volume of the patient. Thus, conventional AEC/ABS systems may undesirably respond to exposure measurements outside the region of diagnostic interest, and in some cases, the exposure measurement locations may lie entirely outside the region of diagnostic interest. This results in exposure measurement errors. Such errors will be large if the attenuation due to the portion of the imaged object which lies in the exposure measurement field differs significantly from the attenuation due to the region of interest.
For example, in imaging examinations involving the extremities of the body, such as the hand, arm, or shoulder, the structure of interest may only partially cover the exposure sensor field. As a result, unattenuated radiation reaches the sensor, and the sensor erroneously includes this radiation in its measurement of the exposure rate. If a radiographic exposure is being conducted under AEC, the exposure is terminated early, and the structure or region of interest will be under-exposed. If a fluoroscopic exposure is being conducted under ABS, the ABS reduces the X-ray tube voltage such that the average image brightness, including the artificially bright portion of the image corresponding to the uncovered portion of the exposure sensor field, approaches the predetermined target brightness. As a result, the structure or region of interest will be artificially darkened. Even if the image of the region of interest remains usable, its diagnostic quality is substantially reduced.
The problem of aligning the structure or region of interest with the exposure sensor field is compounded by the need to perform examinations involving relative motion between the patient and the imaging system. Even if good alignment is initially achieved, relative motion may move the exposure sensor out of alignment with the structure or region of interest, although the structure may remain within the image field. Each relative motion step effectively selects a new structure or region of interest, at which the patient geometry or other characteristics may vary. Thus, at a subsequent imaging position, the selected structure or region of interest may no longer be aligned or sized to correspond with the exposure sensor field. For example, in a peripheral angiography examination of a patient's leg, at a first position, the structure or region of interest may completely fill the exposure sensor field. At a subsequent position, the leg may be thinner, or the orientation of the leg may be different, such that the structure or region of interest does not fill the sensor field, or the sensor field is partially uncovered.